Various optical systems are designed with a purpose of transferring an image of chosen pattern(s) from a pattern-source onto a target workpiece, often multiple times during the manufacturing process. The change in relative orientation and/or position between the target workpiece and the imaging optical system—whether intentional or unintentional—understandably causes the deviations and/or shifts of images projected onto the workpiece from the location(s) of the image(s) determined by design of the process. This begs a question of maintaining such relative orientation during the process or, at least, the ability to determine or track the change of it to compensate for such change in real time.
A non-limiting example of such optical system is provided by an optical metrology system. The optical metrology system can be implemented to qualify the fitness of the already-fabricated workpiece (such as a flatness of a surface or inter-relationship among the structural layers of an optically-complex flat-panel display that came from the manufacturing—process conveyor, or mutual orientation or spatial profiles of optical surfaces of an optical reflector—be it a single surface reflector or an asymmetrically-profiled multi-faceted adaptive optics mirror), or to simply qualify the repositioning/reorientation of the target workpiece (such as a linearly moving stage or a stage defining a tilt or tip of a surface with respect to a reference).
A non-limiting example of a rather specific optical metrology system is offered by a metrology sub-system of a lithographic exposure apparatus (or exposure tool, for short), that is commonly used to transfer images from a reticle, carrying a chosen pattern, onto the workpiece. The workpiece in this case may be (an optionally-repositionable) component that provides an image plane for images of multiple patterns, projected on such component one after another; a substrate (in a rather specific case—a semiconductor wafer); or a component of a panel display fabricated with the use of lithographic processing. A typical exposure apparatus, used for transfer of a pattern from a pattern-source such as a reticle, for example, onto a workpiece of interest (interchangeably—a substrate) may include an illumination source, a reticle stage assembly (that positions a reticle within the apparatus), an optical assembly containing the so-called projection optics, and a workpiece stage assembly (that positions the workpiece).
In a specific case, a measurement or metrology sub-system (that monitors positions of the pattern-source and the target workpiece) often employs an optical contraption that can generally be referred to as an encoder head, and a control system that governs operations of various assemblies to adjust, when required, mutual positioning of the reticle and the target substrate. The geometrical features of patterns transferred from the pattern-source onto the target workpiece are often extremely small, which imposes unparalleled tight requirements on precise positioning of the target workpiece and the pattern-source to ensure high-quality manufacture and/or testing of the already manufactured workpiece.
Accuracy of the measurement/metrology sub-system constantly requires improvement (this is particularly apparent in the case of exposure tool; here, it is partly driven by advances in design of the exposure tool), while relatively small size, simplicity of construction, a need for reduced number of moving parts and high sensitivity remain as practical limitations.
The very kernel of the encoder heads of the related art is structured and built around prismatic elements that include multiple corner cubes. See, for example, US 2013/0128255, US 2015/0276385, US 2014/0049762, to name just a few, the disclosure of each of which is incorporated by reference herein. The disclosure of each of U.S. patent application Ser. Nos. 14/736,118 and 13/796,316 is also incorporated herein by reference.
As appreciated by a skilled artisan and additionally discussed below, the corner-cube-based design necessarily imposes operational shortcoming on the metrology sub-system's structure, among which there are large number of constituent elements/parts, structurally complex input-output optical assemblies, limited size of the optical beam reaching a diffraction grating of the metrology sub-system (which immediately translates to the smaller number of grating lines or grooves available for averaging of optical information), as well as the operational coupling of the sampling of a section of the diffraction grating with measurement beams on the z-position of the grating (that is, a position of the grating along an optical axis of the metrology sub-system).
An implementation of a light-processing portion of the optical system of the metrology sub-system that is freed from the use of thus far inevitable retroreflecting corner-cube elements alleviates the above-identified shortcomings, thereby making the light-encoding operation and/or optical metrology operation robust and less susceptible to errors.